Uncovering category specificity of genital sexual arousal in women

13 Pages·2015·0.46 MB·English

Psychophysiology,00(2015),00–00.WileyPeriodicals,Inc.PrintedintheUSA.
CopyrightVC 2015SocietyforPsychophysiologicalResearch
DOI:10.1111/psyp.12467
Uncovering category specificity of genital sexual arousal in
women: The critical role of analytic technique
CAREY S. PULVERMAN, J. GREGORY HIXON, AND CINDY M. MESTON
DepartmentofPsychology,UniversityofTexasatAustin,Austin,Texas,USA
Abstract
Based on analytic techniques that collapse data into a single average value, it has been reported that women lack
categoryspecificityandshowgenitalsexualarousaltoalargerangeofsexualstimuliincludingthosethatbothmatch
and do notmatch their self-reported sexual interests. These findings may be a methodological artifact of the way in
whichdataareanalyzed.Thisstudyexaminedwhetherusingananalytictechniquethatmodelsdataovertimewould
yield different results. Across two studies, heterosexual (N519) and lesbian (N514) women viewed erotic films
featuringheterosexual,lesbian,andgaymalecouples,respectively,astheirphysiologicalsexualarousalwasassessed
with vaginal photoplethysmography. Data analysis with traditional methods comparing average genital arousal
betweenfilmsfailedtodetectspecificityofgenitalarousalforeithergroup.Whendatawereanalyzedwithsmoothing
regression splines and a within-subjects approach, both heterosexual and lesbian women demonstrated different
patterns of genital sexual arousal to the different types of erotic films, suggesting that sophisticated statistical
techniquesmaybenecessarytomorefullyunderstandwomen’sgenitalsexualarousalresponse.Heterosexualwomen
showedcategory-specificgenitalsexualarousal.Lesbianwomenshowedhigherarousaltotheheterosexualfilmthan
theotherfilms.However,withinsubjects,lesbianwomenshowedsignificantlydifferentarousalresponsessuggesting
that lesbian women’s genital arousal discriminates between different categories of stimuli at the individual level.
Implicationsforthefutureuseofvaginalphotoplethysmographyasadiagnostictoolofsexualpreferencesinclinical
andforensicsettingsarediscussed.
Descriptors: Sex/genderdifferences,Arousal, Analysis/statistical methods,Bloodflow
Category specificity refers to showing genital sexual arousal to & Evers, 1995). In both sexes, the unaroused genitals contain a
erotic stimuli that match one’s sexual preferences, and failing to restinglevelofblood;therefore,baselinearousaltoaneutralstimu-
respondtostimulithatdonotmatchone’ssexualpreferences.Sex- lusmustberecordedpriortothepresentationofeacheroticstimu-
ualstimuliorcuesthatmatchsexualinterestsarereferredtoaspre- lusinlaboratorystudies.Genitalarousalresponseisdefinedasthe
ferred cues, and those that do not match are referred to as changeinarousalovertimefromtheneutraltotheeroticstimulus
nonpreferredcues.Specificityofgenitalsexualarousalistypically andisusuallycalculatedbysubtractingtheaveragearousaltothe
assessed by comparing genital responses to a series of preferred neutralfilmfromtheaveragearousaltotheeroticfilm.
versusnonpreferred sexualcues. Inmen, genital sexual arousal is Instudiesonthecategoryspecificityofgenitalsexualarousal,
typicallymeasuredwithpenileplethysmographyusingamercury- setsofneutralanderoticfilmclipsarepresentedfeaturingcouples
in-rubber strain gauge that tracks penile tumescence (erection). ofdifferentsexualorientations(suchasaheterosexualcoupleora
Penile plethysmography has been shown to be reliable and valid lesbian couple), and subjects self-report their sexual preferences
fortheassessmentofmen’sgenitalsexualarousal(Geer&Janssen, (suchassexualorientation).Arousaltoeachtypeoferoticfilmis
2000;Rosen& Keefe,1978).Inwomen,genitalsexualarousalis averaged and then compared between stimulus categories. Cate-
typically measured with vaginal photoplethysmography, which gory specificity refers to showing significantly higher genital
consists of a tampon-shaped device inserted into the vagina that arousaltoeroticcuesthatmatchone’sself-reportedsexualprefer-
measures vaginal blood volume and vaginal pulse amplitude ences than to cues that do not match one’s sexual preferences.
(VPA), the latter of which has been shown to be a sensitive and Lack of specificity refers to showing equivalent genital sexual
reliableindexofwomen’sgenitalsexualarousal(Laan,Everaerd, arousal to all categories of erotic cues regardless of the match
between the content of the cues and one’s self-reported sexual
preferences.
Therearetwostatisticalmethodscurrentlyusedforevaluating
Address correspondence to: Cindy M. Meston, Ph.D., Department of
thespecificityofgenitalsexualarousal:(1)changeinmeanormax
Psychology, University of Texas at Austin, 108 East Dean Keeton
genital arousal (Bossio, Spape, Lykins, & Chivers, 2014; Bossio,
Street, Stop A8000, Austin, TX 78712, USA. E-mail: meston@psy.
utexas.edu Suschinsky, Puts, & Chivers, 2014; Chivers & Bailey, 2005;
1
2 C.S.Pulverman, J.G.Hixon, andC.M.Meston
Chivers,Seto,&Blanchard,2007;Laanetal.,1995),and(2)cate- sexual assault showgreater genitalsexualarousal tosexual threat
goryspecificityindexorcontrastscore(Chivers,Rieger,Latty,& stimuli than consensual sexual stimuli (Harris, Lalumie`re, Seto,
Bailey, 2004; Peterson, Janssen, & Laan, 2010; Suschinsky, Rice,&Chaplin,2012;Lalumie`re&Quinsey,1994;Lohr,Adams,
Lalumie`re, & Chivers, 2009). Data reduction for these methods &Davis,1997).Menwithpedophilicdisordershowgreatergenital
involvesthebinningofthedatabytime,suchas5-or10-sinter- sexual arousal to sexual stimuli featuring children than those fea-
vals, to reduce the total number of data points, or by using all of turing adults (Blanchard, Klassen, Dickey, Kuban, & Blak, 2001;
therawdataitself. Seto,2004).
Themostwidelyusedmethodofassessingcategoryspecificity The same analytic techniques relying on the average genital
is to compare the change in mean penile circumference or VPA response have demonstrated a lack of category specificity in
between stimulus categories. Comparison of the change in mean women.Womenshowgenitalsexualarousaltobothpreferredand
genital arousal can be completed with analysis of covariance nonpreferred cues for sexual orientation (Chivers et al., 2004,
(ANCOVA) tests on the standardized mean arousal to the erotic 2007;Suschinsky,Lalumie`re,&Chivers,2009),tononhumanpri-
stimuli,usingthestandardizedmeanarousaltotheneutralstimuli matesengagedinsexualactivity(Chivers&Bailey,2005;Chivers
precedingtheeroticstimuliascovariates(Chivers&Bailey,2005; etal.,2007),andstimulidepictingsexualassault(Both,Everaerd,
Laanetal.,1995).Alternatively,someresearchershavecalculated &Laan,2003;Laanetal.,1995;Suschinsky&Lalumie`re,2011b;
the change in mean penile circumference/VPA by subtracting the Suschinskyetal.,2009).Theapparentlackofspecificityinwom-
mean penile circumference/VPA to the neutral film (from either en’sgenitalsexualarousalhasledtomuchdiscussioninthelitera-
theentireneutralfilmortheendportionoftheneutralfilm)from
ture over the past decade on this purported sex difference. If
themeanpenilecircumference/VPAtotheeroticfilm,foreachset
womentrulylackspecificityofgenitalsexualarousal,itmaysug-
offilms.Resultingvaluesarethenstandardizedwithinsubjectsand
gestthatwomen’ssexualarousalismoreflexiblethanmen’s,per-
comparedbetweenstimuluscategories.Manystudieshaveusedthe
haps as a result of broader gender roles for women than for men
changeinmean penile circumference/VPAas their outcome vari-
(Baumeister,2000;Suschinskyetal.,2009).Yet,toourknowledge,
able (Bossio, Spape et al., 2014; Bossio, Suschinsky et al., 2014;
the flexibility explanation for the lack of specificity in women’s
Chiversetal.,2007).Inafewstudies,researchershaveusedmaxi-
genitalsexualarousalhasnotbeentestedempirically.
mumpenilecircumference/VPA(thesinglelargestpenilecircum-
Thepreparationhypothesis,whichalsoattemptstoaccountfor
ference/VPA peak from the erotic film section) rather than mean
the lack of specificity in women’s genital sexual arousal, posits
penilecircumference/VPAtocomputethisstatistic(Suschinsky&
that women show an automatic genital sexual arousal response
Lalumie`re,2011a,b;Suschinskyetal.,2009).
tothepresentationofsexualstimuliasanevolutionaryadaptation
An alternative method for evaluating category specificity is to
toprotecttheirreproductiveorgansifanytypeofvaginalpenetra-
createacategoryspecificityindexorcontrastscore,whichhasalso
tion (consensual or nonconsensual) is to occur (Suschinsky &
been called a standardized penile circumference/VPA difference
Lalumie`re, 2011b). The characterization of women’s genital
score.For thisapproach, change inaveragepenilecircumference/
arousalasanautomaticresponsehasbeensupported(Bassonetal.,
VPA(thestatisticdiscussedabove)iscalculatedbysubtractingthe
2003; Laan & Everaerd, 1995; Laan, Everaerd, Van Aanhold, &
meanarousaltotheneutralfilmfromthemeanarousaltotheerotic
Rebel,1993),withresearchindicatingthatwomen’sgenitalsexual
film for each film category, and the resulting values are standar-
arousalbeginswithinsecondsofthepresentationofavisualerotic
dizedwithinsubjects.Theindexiscreatedbysubtractingthestand-
stimulus(Laan&Janssen,2007).Somewomenwhohaveexperi-
ardized change in mean penile circumference/VPA for one
encedsexualassaultreportgenitalsexualarousalduringtheattack
category from the standardized change in mean penile circumfer-
(Levin & van Berlo, 2004), an experience that can be accounted
ence/VPA for another category, such that positive values indicate
forbythepreparationhypothesis.Empiricalresearchontheprepa-
higher arousal to the first category and negative values indicate
ration hypothesis has been mixed, with some studies supporting
higherarousaltothesecondcategory.Forexample,whenassessing
thisidea(Laanetal.,1995;Suschinsky&Lalumie`re,2011b;Sus-
genital sexual arousal to films featuring heterosexual and lesbian
chinsky et al., 2009), yet others failing to support its predictions
couples,changeinmeanpenilecircumference/VPAtothelesbian
(Dawson,Suschinsky,&Lalumie`re,2013).Dawsonandcolleagues
film issubtracted fromthe change in mean penilecircumference/
examinedgenitalarousaltorepeatedpresentationsofasexualstim-
VPA to the heterosexual film, such that positive values indicate
ulus and found that both men and women showed habituation of
greater arousal to the heterosexual film and negative values indi-
cate greater arousal to the lesbian film. The category specificity genital arousal, contrary to the predictions of the preparation
indexcanbecreatedwithanytwocategoriesofpreferredandnon- hypothesisthatwomen’sarousalresponsewouldfailtohabituate.
preferred erotic stimuli. A number of studies have used the cate- Thenotionthatwomenshowareflexivegenitalarousalresponseto
gory specificity index for data analysis (Chivers et al., 2004; eroticstimulisuggeststhatdifferencesinwomen’sgenitalarousal
Peterson et al., 2010; Suschinsky et al., 2009). Common to both topreferredversusnonpreferredsexualcuesmightbemoresubtle
changeinmeanpenilecircumference/VPAandthecategoryspeci- thandifferencesinmen’sgenitalarousaltoeroticcues.Therefore,
ficityindexisarelianceontheaveragegenitalarousalresponseto testing for category specificity in women’s genital sexual arousal
eachtypeofstimuluscategory,whichcondensestherichpenilecir- mayrequireamoresensitiveanalyticapproach.Althoughtheprep-
cumference/VPA data to a single value and fails to take into aration hypothesis posits an initial reflexive genital arousal
accountthepatternofgenitalarousalovertime. responsetoeroticstimuli,itdoesnotdescribethemaintenanceof
Based on analytic approaches that rely on the average genital thisresponse.Itispossiblethatwomenshowanautomaticgenital
arousaltoeroticcues,studieshaveprovidedevidenceforcategory arousalresponsetoawiderangeofsexualstimuli,yetthemainte-
specificity in men. For example, men only show genital sexual nanceofthatresponseovertimevariesbywomen’ssexualprefer-
arousaltoeroticcuesoftheirpreferredsexualorientation;theydo ences.Thisquestionremainsunanswered,asstudiestendtofocus
not respond genitally to nonpreferred cues (Chivers & Bailey, on the average genital arousal response rather than the pattern of
2005;Chiversetal.,2004).Also,menwithahistoryofcommitting arousalresponseovertime.
Specificity of genitalsexualarousal 3
We propose that the reported lack of category specificity Measures. Physiological data were collected with vaginal photo-
observedinwomenisanartifactoftheanalyticapproachesusedto plethysmographyandelectrocardiography(ECG)equipment.Dur-
evaluatethisconstruct.Toourknowledge,allpreviousstudiestest- ing the stimulus presentation, VPA was sampled at a rate of 200
ingcategoryspecificityinwomenhaveusedsomevariantofaver- samples/second throughout the film, band-pass filtered (0.5–30
age genital arousal for analyses. Average genital arousal is an Hz), and recorded on an MP100 data acquisition unit using the
oversimplification of vaginal photoplethysmograph data, which is AcqKnowledge 3.9.1 software (Biopac Systems, Inc., Santa Bar-
typicallyrecordedat80to200timespersecond,resultinginhun- bara, CA). VPA data were recorded in millivolts. Heart rate was
dredsofthousandsofdatapointspersubject.Viewedinthatcon- assessed with a three-lead ECG with disposable electrodes posi-
text,theaverageisthemostbasicstatisticalindex,iseasilyskewed tioned by the experimenter on the participant’s upper right chest,
by outliers, and fails to reflect variability in data. More sophisti- lowerleftchest,andinnerrightankle.ECGdatawerecollectedto
catedanalyticapproachesthattakealldatapointsintoaccountand identifypeaksintheVPAdataandtohelpwiththeidentification
model data over time might shed new light on the specificity of ofmovementartifacts.
women’sgenitalsexualarousal.Onesuchmethodusessmoothing Subjectivesexualarousalwasassessedwithanadaptedversion
regressionsplines,anonparametricregressiontechniquethatmod- ofthefilmscale(Heiman&Rowland,1983).Theadaptedmeasure
els the trajectory of data over time. In this study, heterosexual includedthreeitems(mentalsexualarousal,sexuallyaroused,and
women’s genital sexual arousal was assessed in response to films sexually turned off) that measured subjective sexual arousal on a
featuring male-female (hereafter referred to as heterosexual), 7-pointLikertscaleindicatingnotatalltointensely.
female-female (referred toas lesbian), andmale-male(referred to Backgroundcharacteristicswerecollectedwithademographics
as gay male) couples engaged in sexual activity and intercourse. questionnaireassessingage,education,relationshipstatusanddura-
Data were analyzed with traditional statistics (change in mean tion,andethnicity/race.
VPA and category specificity index) and smoothing regression SexualorientationwasassessedwiththeKinseyScale(Kinsey,
splines.Smoothingsplinesisolatetwoindependentcomponentsof Pomeroy,Martin,&Gebhard,1953),asingle-itemself-reportmea-
thesexualarousalresponse:overalllevelofarousalandtrajectory sureofsexual orientation. ThisLikert-type scale asksparticipants
ofarousalovertime. Wehypothesize that thetraditionalstatistics toratetheirsexualorientationbasedonboththeirsubjectivereac-
willfailtorevealadifferenceingenitalarousalbetweenfilms,but tionsandovertsexualexperiencesfromexclusivelyheterosexualto
exclusively lesbian. For the purposes of this study, only women
thatthesmoothingsplineanalyseswillshowthatwomenhavean
who identified as exclusively or predominantly heterosexual were
overallhigherlevelanddifferenttrajectoryofarousaltothehetero-
eligibletoparticipate.
sexualfilmthaneitherthelesbianorgaymalefilms.
Stimulus materials. Three10-minaudiovisualfilmswereusedas
stimulusmaterialsinthisstudy.Eachfilmincludedaneutralfilm
Study1
clipandaneroticfilmclip.Theeroticfilmclipsfeaturedahetero-
Method sexual, lesbian, and gay male couple, respectively. Each film
includeda1-minpresentationoftheword“relax,”a3-minneutral
Participants. Participants were adult women recruited from the film showing images of nature and landscapes accompanied by
localcommunity.Recruitmentmaterialsdescribedaresearchstudy classical music, and a 6-min erotic film showing 2 min each of
on women’s sexual arousal, and interested participants called the foreplay, oral sex (cunnilingus in the heterosexual and lesbian
laboratorytocompleteaconfidentialphonescreen.Inclusioncrite- films,andfellatiointhegaymalefilm),andpenetrativesex(vagi-
riaincludedaminimumof18yearsofage,exclusivelyorpredomi- nalintercourseintheheterosexualandlesbianfilms,withastrap-
nantlyheterosexual,andsexuallyfunctional.Sexualfunctionstatus on sex toy used in the lesbian film, and anal sex in the gay male
wasdeterminedduringthephonescreeningbyaskingparticipants film). The films featuring heterosexual and lesbian couples were
if they were experiencing problems with sexual desire, arousal, selectedfromfemale-producederotica,aseroticfilmsproducedby
orgasm,orpain.Onlywomenwhodeniedsexualdysfunctionwere women have been shown to be more appealing to women (Laan,
includedinthestudy.Exclusioncriteriaincludedcurrentlyexperi- Everaerd, Bellen, & Hanewald, 1994). The film featuring a gay
encingmenopauseorperimenopause;historyofsexuallytransmit- malecouplewasselectedfromcommerciallyavailableeroticfilms
ted diseases or current pelvic, vaginal, or urinary tract infection; tomatchtheheterosexualandlesbianfilmsasmuchaspossiblein
historyofapsychoticdisorder;andtakinganyprescriptionmedica- termsofintensityandpositionsofsexualactivity.
tionshowntohavesexualsideeffectsthatcouldaffectthegenital
arousal response, including antidepressants, benzodiazepines, and Procedure. Participants were tested individually by a female
betablockers(Rosen&Kostis,1985;Serretti&Chiesa,2009). researchadministrator.Theadministratorexplainedthestudyproce-
Thecurrentsampleincluded19heterosexualwomenaged18to duresandobtainedinformedconsent.Then,theadministratorposi-
28, with an average age of 21.21 (SD52.64). Fourteen women tioned the ECG pads on the participant and left the room. For the
identified as exclusively heterosexual and five identified as pre- restofthesession,theadministratorandparticipantcommunicated
dominantly heterosexual. The majority of women had completed through an intercom system. The participant inserted the vaginal
some college (79%), 10.5% had completed college, and 10.5% probeandattachedtheECGleadstotheelectropads.Then,thepar-
held an advanced degree. The sample was approximately equally ticipant watched the three erotic films in randomized counterbal-
splitbetweensinglewomen(47%)andwomenwhoweremarried ancedorderasphysiologicalsexualarousalwasassessed.Aftereach
orina committed relationship (53%). Thirty-seven percentofthe film,theparticipantcompletedtheself-reportmeasureofsubjective
sample was Caucasian, 26% was Hispanic, 21% was African sexualarousal,andcompleteddistractortasks(i.e.,mathproblems)
American/Black, 10% identified as biracial, and 5% was Pacific for5mintoallowarousaltoreturntobaseline.Afterallthreefilms,
Islander. theparticipantcompletedthedemographicsquestionnairesandthen
4 C.S.Pulverman, J.G.Hixon, andC.M.Meston
removed the equipment. Participants were compensatedmonetarily categoryspecificityindiceswerecomparedwithrepeatedmeasures
fortheirtime. ANOVAtests.
Dataanalysis. Smoothing regression splines. Smoothing regression splines
Data reduction. VPAwasmeasuredbytheamplitudefromthe are a nonparametric regression technique that models data over
peak to the trough of each waveform. VPA and ECG data were time by constructing a flexible trajectory through all data points.
exported from AcqKnowledge 3.9.3 to Microsoft Excel for proc- Thistrajectorybalancesthefitbetweendatapointswiththenum-
essing.Participantmovementduringthefilm,includingcontraction berofbendsinthetrajectorytoprovideahighlyaccuratemodelof
ofthepelvicmuscles,createsartifactsinthephysiologicaldatathat thedata.Weusedastandardgeneralizedcross-validatedsmoothing
mustberemovedpriortoanalyses(Laanetal.,1995).Movement spline to create a smooth trajectory through all of the VPA data
artifacts were identified and removed by an automatic processing points using the mgcv package (Wood, 2006, 2011) in the open
procedure developed in our laboratory and shown to process data sourceRsoftwareenvironment(RFoundation,2014).Thedegree
more accurately than visual inspection (Pulverman, Meston, & ofsmoothingwasselectedviageneralizedcrossvalidation,which
Hixon,2015).Whentheautomatedprocedurewascomparedwith (among other things) approximates the well-known Akaike Infor-
thetraditionalapproachofvisualinspectionofthedataandmanual mation Criterion (AIC; Akaike, 1974), used for optimal model
removalofartifacts,typicallyutilizedinstudiesoncategoryspeci- selection.
ficity (e.g., Chivers et al., 2007), the automatic procedure was Dataweresampledatarateof200Hz,or200readingspersec-
foundtoidentifyartifactsatacomparableratetovisualinspection ond,whichresultedinadatafilewithapproximately120,000VPA
and to retain more of the original data than the visual inspection readings for each subject (200 readings per second for 10 min of
method (Pulverman et al., 2015). The automatic procedure was film).TheautomaticdataprocessingprocedureidentifiedtheVPA
programmedandexecutedintheopensourceRsoftwareenviron- peaksforeachsubject.Afterprocessing,eachsubjecthadapproxi-
mately600VPApeaks(i.e.,datapoints)perfilm.Inordertomake
ment(RFoundation,2014).
data files more manageable, VPA data were binned by 5-s inter-
RawVPAandECGdatafromExcelwereloadedintotheauto-
vals, resulting in 72 data points for each 6-min erotic film (216
maticprocessingprocedureintheRsoftwareenvironment,which
data points total per subject). To allow for potential differences
identifiestheheartratepeaksintheECGdata.Theprocedurecal-
betweenbothfilmsandsubjectstomanifest,VPAmeanandstand-
culates the mean and standard deviation of heart rate peaks and
ard deviation were calculated across the three film categories
conducts a Bonferroni adjusted (alpha of .05) outlier test on the
together. Data were standardized within each subject using each
time interval between peaks to identify movement artifacts in the
subject’s mean and standard deviation across all three films
ECG data (which also indicate artifacts in the VPA data). These
combined.
spuriousheartratepeaksandassociatedVPAvaluesareremoved
Inordertotestformaineffectsoffilmandsubject,aswellas
fromthedatafile.Then,thevalidheartratepeaksareusedtoiden-
theinteractionoffilmandsubject,codingtermswereconstructed
tifythehighestVPAvaluebetweenheartratepeaks(i.e.,theVPA
to represent each of film type, subject, and their interaction (the
peaks). Next, the procedure uses a standard generalized cross-
unique combination of film and subject). VPA data are time ori-
validatedsmoothingsplinetocreateasmoothtrajectorythroughall
ented, and with time-oriented data, serial dependency is often an
of the VPA data, and residuals between the data points and the
issue. Such serial dependency produces autocorrelated errors,
smoothing spline are calculated. Residuals are used to compute z
whichifunaccountedforwillartificiallyreducestandarderrorsfor
scoresforeachVPApeak,whichareconvertedtoprobabilityval-
modelcoefficientsandoverstatestatisticalsignificance.Toaccount
uesthateachdatapointisanartifact.Datapointswithprobability
for serial dependency, we incorporated a lagged term whereby
valueslessthan1/N,withNbeingthetotalnumberofVPApeaks
each outcome datum serves as an independent predictor of the
in the data file, are identified as artifact. Artifacts are removed
next. Five models were created: (1) lagged term only; (2) lagged
from the data set and the data is modeled again, and this process
term and film type; (3) lagged term and subject; (4) lagged term,
continues until all of the significant artifacts have been removed.
subject,andfilmtype;and(5)laggedterm,subject,filmtype,and
Datafilesforeachsubjectwereprocessedwiththeautomatedarti-
theirinteraction (Film Type 3 Subject).Standard likelihood ratio
factdetectionprocedure.
tests were used to assess the significance of model components.
For example, to test for the subject level effect, the lagged term
Traditional methods. ChangeinmeanVPAfromtheneutralto
only model was compared to the lagged term and subject model,
theeroticfilmwascalculatedforeachfilmbysubtractingthemean
withthe negative two loglikelihood difference assessed againsta
VPAduringtheneutralfilmfromthemeanVPAduringtheerotic
chi-squareddistribution,withdegreesoffreedomequaltothedif-
film.Resultingvalueswerestandardizedwithinsubjectsandcom-
ferenceinthenumberofparameters.
paredwithrepeatedmeasuresanalysisofvariance(ANOVA)tests.
Smoothing splines allow for the examination of two separate
Category specificity index was calculated according to the
characteristicsofthegenitalarousaldata:(1)levelofsexualarousal,
methodusedinpreviousresearch(Chiversetal.,2004;Suschinsky
and (2) trajectory of arousal over time. Level effects describe the
etal.,2009)bysubtractingthechangeinmeanarousaltoonefilm
overallsizeormagnitudeofthegenitalsexualarousalresponse.Tra-
categoryfromthechangeinmeanarousaltoanotherfilmcategory.
jectory over time effects capture the relationship between genital
Inordertocomparearousaltothethreefilms,threeseparatecate-
arousalandtime,independentoflevelofarousaleffects.
gory specificity indices were computed: the heterosexual/lesbian
contrast, the heterosexual/gay contrast, and the lesbian/gay con-
Results
trast.Forthesestatistics,positivevaluesindicatehigherarousalto
the first filmcategory andnegative valuesindicate higher arousal Traditional methods. Change in mean VPA from the neutral to
to the second film category. For each category specificity index, eroticfilmsectionswascomputedandstandardizedwithinsubjects
the mean, standard deviation, and range are reported. The three for the heterosexual (M5-.002, SD5.98), lesbian (M5-.08,
Specificity of genitalsexualarousal 5
SD5.80), and gay male (M5.08, SD5.71) films. A repeated
measures ANOVA test on change in mean VPA found no
difference in genital sexual arousal to the three erotic films,
F(2,36) 5.12, p5.88. This finding replicated those of previous
studies (Bossio, Suschinsky et al., 2014; Chivers & Bailey, 2005;
Laanetal.,1995),whichalsofailedtodetectcategoryspecificity
in women’s genital sexual arousal with change in mean VPA
analyses.
Forthecategoryspecificityindexanalyses,higherscoresindicate
greaterarousaltothefirstfilmcategoryandnegativescoresgreater
arousal to the second film category. These indices were calculated
with the standardized change in mean arousal values for the three
filmsdescribedinthepreviousparagraph.Theheterosexual/lesbian
contrastrangedfrom1.98to22.00,(M5.08,SD51.64).Thehet-
erosexual/gay contrast ranged from 2.00 to 22.00 (M5-.09,
SD51.51). The lesbian/gay contrast ranged from 1.62 to 21.83
Figure 1.Smoothing regression splines showing the overall level and
(M5-.17,SD51.15).ArepeatedmeasuresANOVAtestfoundno
trajectory of standardized genital sexual arousal over time to the three
differencebetweenthecategoryspecificityindicesforthethreecon-
eroticfilmsforheterosexualwomenasagroup.Womendemonstrateda
trasts,F(2,36)5.18,p5.84.Thisfindingreplicatedthoseofprevi-
higher overall level of arousal to the heterosexual film than the other
ousstudies (Chiverset al., 2004; Peterson et al., 2010; Suschinsky two films, and the trajectories of arousal were significantly different
etal.,2009),whichalsofailedtodetectcategoryspecificityofgeni- between all three films. VPA was recorded in millivolts and standar-
talsexualarousalusingcategoryspecificityindexstatistics. dizedwithinsubjects.Residualstandarddeviation50.65.
Smoothingregressionsplines. malefilmsbothincreasedovertime(seeFigure1),theyweresig-
Level of sexual arousal. Therewasasignificantmaineffectof nificantlydifferent,indicatingthattheyincreasedatdifferentrates.
filmonthelevelofarousal,v2(2.00)511.12,p<.01,withaneffect By visual inspection of Figure 1, it appears that the trajectory of
size of R25.003. Pairwise comparisons revealed that the level of arousaltothelesbianfilm,inwhicharousalincreasedfrombaseline
arousaltotheheterosexualfilmwassignificantlyhigherthanthelevel but then dropped off, demonstrates that arousal decreased as
tothelesbianfilm,v2(1.00)55.76,p<.052,R25.002,andtothe womenwereexposedtomoreofthefilmovertime.
gay male film, v2(1.00)58.84, p<.01, R25.003, indicating that Additionally, there was a significant main effect of subject on
heterosexualwomenshowedgreatergenitalarousaltotheheterosex- trajectory of arousal, v2(21.17)561.13, p<.001, R25.02, sug-
ual film (preferred film) than either the lesbian or gay male films
gesting that the trajectories of arousal were different from one
(nonpreferred films). There was no difference in women’s level of
womantoanother.
arousal between the lesbian and gay male films, v2(1.00)5.91,
TherewasalsoasignificantinteractioneffectofSubject3Film
p5.34(seeFigure1). on trajectory of arousal, v2(135.93)51044.89, p<.001, R25.22,
Themaineffectofsubjectonthelevelofarousalwasnotsig-
suggestingthatthetrajectoriesofwomen’sarousaldifferedbetween
nificant, v2(18.00)51.43, p51.00, as would be expected due to
filmswhenexaminedwithinsubjects.Theinteractioneffectcanbe
standardization.
illustratedbycomparingFigure1,showingthearousalresponseof
There was a significant level effect for the interaction of
allwomen,toFigure2,showingthearousalresponseoftwoindivid-
Subject 3 Film, v2(36.00)5401.46, p<.001. The interaction ualwomen. Againthe interaction effect (R25.22) was larger than
indicates that the level of women’s arousal differed significantly the effect of either film type (R25.01) or subject (R25.02) inde-
betweenfilmswhenexaminedwithinsubjects.Notably,theinter-
pendently,suggestingthatbothindividual subjectcharacteristicsas
action effect (R25.07) was larger than the main effect of film
well as stimulus category should be considered when examining
(R25.003) on level of arousal. This suggests that, although the
women’sgenitalsexualarousalresponse.Womenshowedcategory-
filmsthemselvesaffectedarousallevel,theeffectoffilmonlevel
specificgenitalsexualarousalbothintermsoftheiroveralllevelof
of arousal was even stronger when compared within subjects. In
arousalandthetrajectoryoftheirarousalovertime.
other words, the variations in response from one film to another
withinindividualsubjectswassubstantiallygreaterthanthevaria-
Subjective sexual arousal. Althoughnotthefocusofthisstudy,
tionfromonefilmtoanotheringeneralacrossallsubjects.
subjectivesexualarousalwasassessedtoensurethatwomenfound
Trajectory of sexual arousal. Comparing the trajectories of thefilmssubjectivelysexuallyarousing.Subjectivesexualarousal
genitalarousaltothethreefilms(independentoflevel),therewasa responseswerescoredfrom0–18,withhighernumbersindicating
significant main effect of film, v2(4.10)539.42, p<.001, highersexualarousal.Subjectivesexualarousalwasreportedtothe
R25.01. Pairwise comparisons revealed that the trajectory of heterosexual film (M512.00, SD53.20), the lesbian film
arousaltotheheterosexualfilmwassignificantlydifferentfromthe (M56.22, SD54.20), and the gay male film (M55.72,
trajectorytothelesbianfilm,v2(3.13)522.23,p<.001,R25.008, SD53.59). Subjective arousal scores were similar to scores
and to the gay male film, v2(2.00)522.28, p<.001, R25.008. reportedbysexuallyfunctionalheterosexualwomeninotherstud-
Thetrajectoryofarousaltothelesbianfilmwasalsosignificantly ies in our laboratory, suggesting that the stimuli films were suffi-
differentfromthetrajectorytothegaymalefilm,v2(3.24)535.18, ciently psychologically sexually arousing to subjects. Subjective
p<.001,R25.01.Thissuggeststhatwomenshoweddifferentpat- arousal scores were compared between films with repeated meas-
terns of genital sexual arousal to all three of the erotic films. uresANOVAtests.Therewasasignificantdifferenceinsubjective
Although the trajectories of arousal to the heterosexual and gay sexualarousalbetweenfilms,F(2,34)526.83,p<.001,effectsize
6 C.S.Pulverman, J.G.Hixon, andC.M.Meston
Figure2.Thesesetsof graphsshowthegenitalarousalresponsestothethreefilmsovertimefortwoindividualheterosexual subjects(heterosexual
SubjectsAandB).Theverticaldashedredlinesindicateachangeinthecontentoftheeroticfilms,whichincluded2minofforeplay(0to120s),
2minoforalsex(120to240s),and2minofpenetrativesex(240to360s).Graphsofindividualresponsesillustratetheinteractioneffect:women
showed even greater variability in their arousal responses when examined within subjects (i.e., each woman just compared to herself) than when
examinedtogetherasagroup(asinFigure1).Smoothingsplinesrevealnuanceddifferencesingenitalresponsethataremaskedinthegroupanalyses.
There was a significant interaction between subject and film for both level of genital sexual arousal and trajectory of genital sexual arousal in the
smoothingsplineanalyses.Greendashedlinesrepresent95%confidenceintervalsaroundthesmoothingsplines.VPAwasrecordedinmillivoltsand
standardizedwithinsubjects.Residualstandarddeviation50.55.
g25.61.Pairwisecomparisonsrevealedasignificantdifferencein Discussion
subjectivesexualarousalbetweentheheterosexualfilmandtheles-
bianfilm,F(1,17)532.01,p<.001,andbetweentheheterosexual The aim of this study was to examine category specificity in
filmandthegaymalefilm,F(1,17)537.73,p<.001.Thesample women using an advanced analytic technique: smoothing regres-
reportedsignificantlyhighersubjectivesexualarousaltotheheter- sion splines. We examined heterosexual women’s genital arousal
osexualfilmthaneitheroftheothertwofilms. responses to preferred and nonpreferred erotic films with the
Specificity of genitalsexualarousal 7
traditionalchangeinmeanVPAandcategoryspecificityindexsta- Kinsey Scale toqualifyfor this study. Bisexual-identified women
tistics and failed to find differences in average arousal between wereexcludedfromthestudy.
films.Whenweexaminedthesamedatawithsmoothingregression ThesampleforStudy2included14lesbianwomenaged18to
splines,wefoundthatheterosexualwomendemonstratedahigher 47,withanaverageageof26.07(SD58.38).Sevenwomenidenti-
overall level of genital sexual arousal to the heterosexual erotic fied as exclusively lesbian and seven identified as predominantly
filmthantothelesbianandgaymaleeroticfilms(seeFigure1).In lesbian.Forty-threepercentofthesamplehadcompletedsomecol-
termsofthetrajectoryorpatternofarousal,womenshowedsignifi- lege, 43% had completed college, and 14% held an advanced
cantlydifferenttrajectoriesofarousaltoeachfilm.AsseeninFig- degree. There were slightly more single women in the sample
ure1,arousaltotheheterosexualandgaymalefilmsincreasedina (57%)thanwomenmarriedorinacommittedrelationship(43%).
linear pattern across the film presentation (although at a higher Fifty-seven percent of the sample was Caucasian, 21% was His-
level to the heterosexual film), but arousal to the lesbian film panic,14%wasAsian,and8%wasAfricanAmerican/Black.
increasedinitiallyandthendecreased.Asthedifferencesbetween
Measures, stimulus materials, procedure, data analysis. Self-
theselevelsandtrajectoriesofarousalweresignificantlydifferent
report measures, erotic films, study procedure, and data analysis
between the heterosexual film and the other two films, we con-
methodswereidenticaltotheproceduresdescribedinStudy1.
cludedthatheterosexualwomenhavecategory-specificgenitalsex-
ual arousal when measured with smoothing regression splines.
Theseresultssuggestthatitisnecessarytoexaminebothleveland Results
trajectoryofarousalinordertoevaluatethisdataaccurately.
Traditional methods. Change in mean VPA from the neutral to
Inaddition to the influence ofthe content of erotic films (i.e.,
eroticfilmsectionswascomputedandstandardizedwithinsubjects
sexualorientationofcouplepresented)onwomen’sgenitalarousal,
for the lesbian (M5-.40, SD5.77), heterosexual (M5-.008,
we found that the individual subject was an even more important
SD5.75), and gay male (M5.41, SD5.80) films. A repeated
determinant of arousal response. For both the level and trajectory
measures ANOVA test on change in mean VPA found no differ-
ofgenitalsexualarousal,wheneachwoman’sarousaltoeachfilm
ence in genital sexual arousal between films, F(2,26)52.58,
wascomparedtoherownarousaltotheothertwofilms,therewere p5.10.Similartopreviousstudies(Chiversetal.,2007),wefailed
largereffectsthanwhenwomen’sarousalresponseswerecollapsed to detect category specificity of lesbian women’s genital sexual
into a single group and compared between films. In other words, arousalusingchangeinmeanVPAstatistics.
the arousal response depended upon the individual characteristics Inthecategoryspecificityindexanalyses,positivescoresindi-
ofeachwoman.Theenhancedsensitivityofsmoothingsplinescan cate greater arousal to the first film category and negative scores
revealeffectsacrossallwomenasagroup,aswellaswithineach greaterarousaltothesecondfilmcategory.ForStudy2,category
woman individually, ultimately allowing for more comprehensive specificity indices were calculated in comparison to the lesbian
dataanalysis. filmsuchthatforbothcomparisonsthatincludedthelesbianfilm
Inanefforttofurtherexaminethisnotablefinding,werepeated positivescoresindicategreaterarousaltothelesbianfilm.Theles-
thestudywithlesbianwomen.Testingforcategoryspecificityina bian/heterosexual contrast ranged from 1.73 to 21.95, (M5-.39,
secondgroupofwomenprovidesadditionalexplorationoftheeffi- SD51.29). The lesbian/gay contrast ranged from 1.86 to 22.00
cacyofournewanalyticapproach.Lesbianwomenwereselected (M5-.81,SD51.38).Theheterosexual/gaycontrastrangedfrom
asthesecondsamplebecause,similartoheterosexualwomen,itis 1.88 to 21.92 (M5-.42, SD51.35). A repeated measures
possibletoclearlydefinecategoriesofpreferredandnonpreferred ANOVAtestfoundnodifferencesbetweenthecategoryspecificity
erotic cues for lesbian women. Previous studies on the category indicesforthethreecontrasts,F(2,26)5.54,p5.59.Thisfinding
specificity of women’s genital sexual arousal have included sam- replicatedthoseofpreviousstudies(Chiversetal.,2004;Peterson
plesofbothheterosexualandlesbianwomen(Chiversetal.,2007; etal.,2010),failingtodetectdifferencesingenitalarousaltodif-
Petersonetal.,2010).Wehypothesizedthat,liketheheterosexual ferenttypesoferoticfilmsforlesbianwomenusingcategoryspeci-
women, lesbian women would show a higher overall level of ficityindexanalyses.
arousal and different trajectory of arousal to their preferred erotic
film (lesbian film) than to their nonpreferred films (heterosexual Smoothingregression splines.
andgaymalefilms). Level of sexual arousal. There was a significant main effect
offilmonlevelofarousal,v2(2.00)56.21,p<.05,withaneffect
size of R25.002. Unexpectedly, pairwise comparisons revealed
Study2
thatthelevelofarousaltothelesbianfilmwassignificantlylower
Method thanthelevelofarousaltotheheterosexualfilm,v2(1.00)55.56,
p<.05,R25.003.Therewerenodifferencesinlevelofarousalto
Participants. Participants were lesbian adult women recruited thelesbianandgaymalefilms,v2(1.00)51.96,p5.16,orthehet-
from the local community. Recruitment materials described a erosexual and gay male films, v2(1.00)51.37, p5.24. Lesbian
research study on lesbian women’s sexual arousal, and interested womenshowedahigherlevelofgenitalarousaltotheheterosexual
participantscalledthelaboratorytocompleteaconfidentialphone filmthanthelesbianfilm(see Figure 3).Thisunexpectedfinding
screen. Recruitment materials were presented in the same venues willbeexaminedintheDiscussion.
asin Study1,withthe addition oflocations known to attract les- Themaineffectofsubjectonthelevelofarousalwasnotsig-
bian women such as sexuality counseling centers and female- nificant v2(13.00)52.81, p51.00, as would be expected due to
focused bookstores. Inclusion and exclusion criteria for Study 2 standardization.
were identical to Study 1, except that heterosexual women were There was a significant level effect for the interaction of
excluded and lesbian women included in the study. Women were Subject 3 Film, v2(26.00)5258.25, p<.001, R25.08. When
requiredtoidentifyasexclusivelyorpredominantlylesbianonthe examined within subjects, the level of each woman’s arousal
8 C.S.Pulverman, J.G.Hixon, andC.M.Meston
insubjectivesexualarousalbetweenfilms,F(2,26)53.47,p<.05,
effect size g25.21. Pairwise comparisons revealed a significant
difference in subjective sexual arousal between the lesbian and
gay male erotic films, F(1,13)56.40, p<.05, but no difference
in arousal between the lesbian and heterosexual erotic films,
F(1,13)51.30,p5.28.
Discussion
Examinationoflesbianwomen’sarousalresponseswithtraditional
changeinmeanVPAandcategoryspecificityindexstatisticsfailed
tofindmeaningfuldifferencesinaveragearousalbetweenthethree
films. Smoothing regression splines revealed that lesbian women
showedahigheroveralllevelofgenitalsexualarousaltotheheter-
osexualfilmthantothelesbianorgaymalefilms.Thiscontradicts
ourcurrentconceptionofcategoryspecificity,thathighestarousal
Figure 3.Smoothing regression splines showing the overall level and will occur in response to films that match sexual preferences. Of
trajectory of standardized genital sexual arousal over time to the three course, these findings may indicate that lesbian women report a
erotic films for lesbian women as a group. Contrary to our hypotheses, sexualpreferenceforlesbiansexualactivity,butareactuallymore
lesbian women showed an overall lower level of arousal to the lesbian
genitallysexuallyarousedtovisualdepictionsofheterosexualsex-
filmthantheheterosexualfilm.Therewerenodifferencesinthetrajec-
ualactivity.However,itisimportanttonotethat,whenexamined
tories of arousal to the three films for the lesbian women. VPA was
withinsubjects,lesbianwomenshowedsignificantlydifferentlev-
recorded in millivolts and standardized within subjects. Residual stand-
elsandpatternsofarousaltothethreefilms,indicatingthat,atan
arddeviation50.72.
individuallevel,genitalarousaldiscriminatedbetweenthedifferent
typesofstimuli.
responsetothefilmsdiffered.Theinteractioneffect(R25.08)
Examinationofthetrajectoryofarousalwithinsubjectshelpsto
was larger than the main effect of film (R25.002) on level of
explainthesenuanceddifferencesinpatternsofarousalforlesbian
arousal.Theinteractionsuggeststhattheeffectoffilmonlevel
women.Forexample,thesetofthreegraphsforLesbianSubjectA
of arousal was larger when examined within subjects than
inFigure4showsthegenitalarousalresponsetothethreefilmsfor
whenexaminedacrossallsubjectstogether.
one individual lesbian subject. As a reminder, each erotic film
Trajectory of sexual arousal. Comparing the trajectories of included2mineachofforeplay,oralsex,andpenetrativesex.Vis-
genitalarousaltothethreefilms(independentoflevel),therewas ualinspectionofthesegraphsrevealsthatthissubject’sarousalto
nomaineffectoffilm,v2(3.00)52.70,p5.44.Thetrajectoriesof the heterosexual and gay male films decreased over time and
arousaltothethreefilmsdidnotdiffersignificantly(seeFigure3). arousal to the lesbian film increased over time. The increase in
There was no main effect of subject on trajectory of arousal, arousalacrossthelesbianfilmisconsistentwithpreviousresearch
v2(14.00)513.96, p5.45, indicating that when just comparing indicating that women show greater genital sexual arousal in
arousalwithineachwomantherewerenosignificantdifferencesin responsetoeroticstimuliofhigherintensity(i.e.,highertosexual
thetrajectoriesofherarousal. intercourse than oral sex, foreplay, or solitary masturbation), an
TherewasasignificantinteractioneffectofSubject3Filmfor effectthathasbeentermedactivityspecificity(Bossio,Suschinsky
trajectory of arousal, v2(106.55)51446.23, p<.001, R25.35, etal.,2014;Both,Boxtel,Stekelenburg,Everaerd,& Laan,2005;
suggesting that when examined within subjects the trajectories of Chivers et al., 2007; Suschinsky, Bossio, & Chivers, 2014). Yet,
lesbian women’s arousal to the three films differed significantly. the smoothing spline analyses reveal that this subject’s arousal
Therewasnomaineffectoffilmontrajectoryofarousal,butthe responsetothelesbianfilmwasmorecomplexthanasimplelinear
interactionofsubjectandfilmshowedalargeeffectontrajectory increase across the course of the film as the sexual activity pre-
ofarousal,indicatingthat,withinsubjects,lesbianwomenshowed sented intensified. Her arousal increased slightly during the fore-
differentpatternsofarousaltothethreefilms.Lesbianwomendid playandoralsexportionsofthefilm,increasedmoredramatically
notshowahigheroveralllevelorvariabletrajectoryofarousalto duringthebeginningofthevaginalpenetrationportionofthefilm,
thelesbianfilmaswehadhypothesized,yettheystillshoweddif- andthendecreasedtowardtheendofthevaginalpenetrationpor-
ferent levels and patterns of arousal to the three films. Although tionofthefilm.Theapplicationofsmoothingsplinestoindividual
thisfindingdoesnotfitwithourcurrentconceptualizationofcate- subjects’arousaldataenablesustoidentifythesenuanceddifferen-
gory specificity, it may be important for understanding lesbian ces in individual arousal response that are masked in the group
women’sgenitalarousalresponse. analysesasthegroupanalysescollapseacrossindividualdifferen-
ces,whichthe currentresults indicatearemeaningfulandstatisti-
Subjective sexual arousal. Subjective sexual arousal responses callysignificant.Asthisisthefirstopportunityscientistshavehad
were scored from 0 to 18, with higher numbers indicating higher toexamineindividualarousaltrajectoriesovertime,ourinterpreta-
subjectivesexualarousal.Lesbianwomenreportedsubjectivesex- tions of the meaning of these results are speculative, and future
ualarousaltothelesbianfilm(M512.11,SD54.39),heterosex- researchwillbeneededtoclarifytherelationshipbetweenindivid-
ual film (M510.54, SD52.31), and gay male film (M58.50, ualdifferencesinarousalresponseandsexualpreferences.Avari-
SD55.67). These subjective arousal scores are similar to scores etyoffactorscouldhave affectedthiswoman’sresponse,suchas
reported by sexuallyfunctional lesbianwomen inother studies in the sexual activities depicted in the erotic films (Chivers et al.,
our laboratory, suggesting that the stimuli films were sufficiently 2007; Suschinsky et al., 2014), and characteristics of the actors
sexuallyarousingtothesubjects.Therewasasignificantdifference such as their physical attractiveness, race and ethnicity, and
Specificity of genitalsexualarousal 9
Figure4.Thesegraphsillustrategenitalarousalresponsestothethreefilmsovertimefortwoindividuallesbiansubjects(lesbianSubjectsAandB).
Theverticaldashedredlinesindicateachangeinthecontentoftheeroticfilm,whichincluded2minofforeplay(0to120s),2minoforalsex(120
to 240 s), and 2 min of penetrative sex (240 to 360 s). Graphs of individual responses illustrate the interaction effect: lesbian women showed even
greatervariabilityintheirarousalresponseswhenexaminedwithinsubjects(i.e.,eachwomanjustcomparedtoherself)thanwhenexaminedtogether
asagroup(asinFigure3).Smoothingsplinesrevealnuanceddifferencesingenitalresponsethataremaskedinthegroupanalyses.Therewasasig-
nificant interaction between subject and film for both level of genital sexual arousal and trajectory of genital sexual arousal in the smoothing spline
analyses.Dashedlinesrepresent95%confidenceintervalsaroundthesmoothingsplines.VPAwasrecordedinmillivoltsandstandardizedwithinsub-
jects.Residualstandarddeviation50.55.
similarity to the participant and/or her past sexual partners. It calls for future research on the influence of various elements of
appearsthat,atanindividuallevel,lesbianwomen’sgenitalsexual eroticstimulionlesbianwomen’sgenitalsexualarousalresponse.
arousal response discriminates between different elements of the
eroticstimulus,suchascontentandactorcharacteristics,atamuch
General Discussion
more nuanced level than previously known. Although individual
differencesinthelesbianwomen’sgenitalresponsescannotexplain The aim of these studies was to determine whether smoothing
thelackofcategoryspecificityobservedinthisgroup,thesediffer- spline analyses would yield different results for the specificity of
ences suggest that arousal is affected by a number of factors and women’s genital sexual arousal than the traditional analytic
10 C.S.Pulverman, J.G.Hixon, andC.M.Meston
method, which focuses on average arousal response. For both stimuli to a much larger degree than previously understood (e.g.,
groups of women, our findings supported previous research (e.g., Figures 2 and 4). Women may respond uniquely to a number of
Chivers,2005),failingtodetectcategoryspecificityineithergroup characteristicsoftheeroticstimulussuchasthesettingofthefilm,
ofwomenusingthetraditionalchangeinmeanVPAandcategory sexualactivitiesdepicted,sexualpositionspresented,andphysical
specificity index analyses. However, for the heterosexual women, attractiveness of the actors. Aside from the robust finding that
smoothing spline analyses indicated that this group showed a women show higher arousal tosexual activity of greater intensity
higher level and different patterns of arousal to the heterosexual (Bossio,Suschinskyetal.,2014;Chiversetal.,2007),therelative
film than to either the lesbian or gay male films (see Figure 1). influence of factors such as setting, sexual positions, and actor
Smoothing splines revealed that lesbian women showed different characteristicsonwomen’sgenitalarousalresponse areunknown.
levelsandpatternsofarousaltothethreefilms(althoughthesepat- Future studies are needed to determine the stimuli characteristics
ternsdidnotnecessarilyreflecttheirself-reportedsexualpreferen- mostsexuallyarousingtowomeninordertodevelopbetterquality
ces;seeFigure3).Smoothingsplinesalsoallowfortheevaluation eroticstimuliforresearchstudies.Preferencesforstimulicharacter-
ofwithin-subjectseffects,theinfluenceofeachindividualsubject isticsmayvaryalongdemographiclinessuchasage,sexualorien-
onarousalresponse.Forboththeheterosexualandlesbianwomen, tation, racial or ethnic background, and past sexual experiences
differences in arousal to the films were strongest when assessed such as sexual abuse. Previous studies of erotic stimuli develop-
withinsubjects(seeFigures2and4).Thissuggeststhatindividual ment have indicated that heterosexual women prefer female-
characteristicsofeachwomanplayanimportantroleinhergenital produced erotica (Laan et al., 1994), and that heterosexual and
sexual arousal response. Though one previous study with hetero- bisexual women prefer stimuli featuring vaginal intercourse in
sexualwomenfoundthattheyshowedhighergenitalsexualarousal male-on-femalesexualpositions(Woodardetal.,2008).Reviewof
tostillimagesofarousedmalegenitalsthantoimagesofaroused theliteraturerevealsanotablelackofresearchonidealeroticstim-
female genitals (Spape, Timmers, Yoon, Ponseti, & Chivers, uliforlesbianwomen.Assmoothingregressionsplinesallowfora
2014),toourknowledgeoursisthefirststudytodemonstratecate- moresensitiveassessmentofgenitalarousalresponsethanwaspre-
gory specificity in women’s genital sexual arousal in response to viously possible, future research could examine women’s sexual
eroticfilms. preferencesatamuchmoremicrolevel.Betterqualitystimulithat
The identification of category specificity in women’s arousal aremorelikelytoleadtoahighlevelofarousalinthelaboratory
with smoothing splines highlights the importance of using appro- couldimprovesexresearchexperiments.Itmightalsobepossible
priateanalytictechniquestoexaminegenitalarousaldata.Physio- tocustomizepreferredor“ideal”stimuliforeachindividualsubject
logical measures like vaginal photoplethysmography provide prior to running a study. Although this suggestion brings up con-
continuousassessment,leadingtoarichsetofdatathatcantella cernsaboutstandardization,ifeachwomanwasequallyarousedto
story over time. Using the average to evaluate genital arousal her customized film(s) in pretesting, this approach could possibly
response fails to take advantage of the continuous nature of this leadtomoreecologicallyvalidtestingresults.
data. Smoothing splines provide the opportunity to examine two Another way in which smoothing splines could improve sex
componentsofgenitalarousaldata:overalllevelofsexualarousal research experiments is by comparing arousal responses between
and trajectory of arousal over time. Overall level represents the different manipulations or treatments. In the same way that
strength of the arousal response. Trajectory or pattern over time smoothing splines reflect variations in women’s genital arousal
shows the behavior of arousal across the testing session. As response to different categories of erotic stimuli, they could also
smoothingsplineanalysesarecompletelynewtophysiologicalsex reflect differences in arousal response to levels of a manipulation
research,the meaningoflevelof genitalarousal andtrajectoryof ortreatmentforsexualarousaldysfunction.Pharmacologicaltreat-
genitalarousaltothefieldwillundoubtedlyberefinedwithfurther ments for female sexual dysfunction provide a case example.
studyandresearchusingthisanalytictechnique. Smoothing splines could be applied to questions about response
To date, the literature on vaginal photoplethysmography as a latencyandappropriatedosage.Informationonthelengthoftime
tool for the assessment of genital sexual arousal indicates that ittakesdrugstoaffectsexualarousalwouldbehighlyimportantto
women’sarousalincreasesinresponsetoeroticfilms(Laanetal., thedevelopmentofaneffectivedrugforfemalesexualarousaldys-
1995),butdoesnotrevealinformationaboutwomen’ssexualpref- function. Traditional methods of analyzing women’s genital
erences.Priorliteratureoncategoryspecificityhascalledthelack arousalresponserelyingontheaverageresponsedonotaffordthe
ofspecificityinwomen’sarousalarobusteffect(Bossio,Suschin- opportunitytoexaminethetimetrajectoryofeffects.Todate,the
skyetal.,2014;Chivers,2005).Theinabilityofvaginalphotople- Food and Drug Administration has not approved any drugs for
thysmographytodescribesexualpreferencesisa commonlycited female sexual dysfunction (Clayton et al., 2010), and access to
drawback of this assessment tool (Suschinsky et al., 2009). The more detailed information on the impact of drugs on women’s
current study suggests that vaginal photoplethysmography may arousal over time could shed new light on this influential area of
reveal more about women’s sexual preferences than previously research.
believed. The potential for vaginal photoplethysmography to Smoothing regression splines could also be applied to data on
describewomen’ssexualpreferencesopensupnewresearchappli- continuoussubjectivesexualarousalinordertoevaluateconcord-
cationsaswellaspotentialopportunitiesfortheuseofplethysmog- anceofgenitalandsubjectivesexualarousal.Self-reportedpsycho-
raphyinclinicalandforensicsettings. logicalorsubjectivesexualarousalisoftencollectedalongsidethe
assessment of genital sexual arousal in sex research experiments.
Subjective sexual arousal is typically assessed with a lever-like
Research Applications
instrumentthatsubjectsadjustcontinuouslyduringthefilmpresen-
Smoothing splines provide a highly sensitive and individualized tation to indicate changes in subjective sexual arousal (Rellini,
assessmentofgenitalarousaldataintheformoftwoindices,level McCall, Randall, & Meston, 2005). Concordance provides infor-
andtrajectory.Resultsfromthecurrentstudiessuggestthatwom- mationonthemeaningofself-reportedsexualarousal,whichisrel-
en’s genital response discriminates between features of erotic evant to both models of the human sexual response as well as